Monday, June 26, 2017

Causality is essential to physics

I mentioned Massimo Pigliucci's attack on causality. He has now closed comments, so I respond here (ignoring his usual ad hominem attacks).

He argues that causality is important in all the sciences except for fundamental physics, where it is not because of the following chain of reasoning:

* The equations of fundamental physics have a time reversal symmetry.
* Microscopic physics obeys those equations, and hence has no arrow of time.
* Entropy has an arrow of time, but that is classical physics, and hence not fundamental.
* Without a fundamental arrow of time, there is no way to say one thing causes another.

This is just wrong on every level. The equations of physics do have time reversal asymmetries. Even system with time reversal symmetric equations have physics showing an arrow of time. Entropy increases on both the quantum and classical levels, and is as fundamental as anything. The soft sciences prove causality often, without using any arrow of time.

As commenter Coel explains, quantum mechanics is an irreversible theory. Every observation is irreversible. Decoherence is irreversible. CP violating weak interactions are irreversible.

Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true. Yet at the macroscopic level it often appears that this is not the case: there is an obvious direction (or flow) of time.

This statement is artfully misleading, as there is also an obvious direction of time at the microscopic level.

You might see a neutron decay into a proton, electron, and (anti-)neutrino. You never see a proton, electron, and anti-neutrino all coming together to make a neutron. Likewise, other nuclear reaction have an obvious direction of time.

Wave equations often have time reversal symmetries, but the observed waves do not. Waves go forward in time from initial conditions, and this is often obvious by looking at the wave.

All of this does not really have much to do with causality. A medical paper might have data showing that smoking causing lung cancer, but it does not need an arrow of time to reach the conclusion. There would be causality even if all the laws of physics were time symmetric.

I am not just blaming MP here, as he says he is just reciting conventional wisdom among philosophers. If so, then philosophers do not know the first things about physics.

17 comments:

Massimo Pigliucci quotes, "Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true. Yet at the macroscopic level it often appears that this is not the case: there is an obvious direction (or flow) of time."

When a photon blackens a film grain in a photographic film, this is a microscopic process. How often has Pigliucci observed a blackened grain of film turning white again and emitting a photon?

Roger,You really need a refresh on philosophy yourself. You are playing very fast and lose with 'If so...' stacked on top of an assumption, and employing blanketing statements that draw no distinction between some and all. Some philosophers do not understand physics is true.All philosophers do not understand physics is absurdly false, and you know better.A great number of so called physicists don't exactly know physics very well either, and many subscribe to some flavor of metaphysical mathematical Platonism.

Massimo Pigliucci quotes, "Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true. Yet at the macroscopic level it often appears that this is not the case: there is an obvious direction (or flow) of time."

A photon is described by a spherically-symmetric probabilistic distribution expanding at the rate of c. Massimo Pigliucci writes, "Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric." Could Massimo Pigliucci please share the experiment that shows the photon as a spherically-symmetric probabilistic distribution COLLAPSING at the rate of c?

In the double-slit experiment, the photon leaves the source, passes through the slits, and leaves a point-like mark on the screen.

Massimo Pigliucci quotes, "Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true. Yet at the macroscopic level it often appears that this is not the case: there is an obvious direction (or flow) of time."

Could Massimo Pigliucci please share the experiment where the point-like mark on the screen becomes a photon, passes backwards through the two slits, and hits the source, adding to its energy?

Huygens' Principle states: "every point which a luminous disturbance reaches becomes a source of a spherical wave; the sum of these secondary waves determines the form of the wave at any subsequent time. " https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle

Massimo Pigliucci quotes, "Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true."

Could Pigliucci please provide the experiment which demonstrates Huygens' Principle acting in reverse, where said spherical wave collapses inwards and becomes a point, and said points form another sphere and collapse inwards, becoming a point?

Neutrons decay as given here: https://en.wikipedia.org/wiki/Free_neutron_decay

The Neutron decays into a proton, electron and electron antineutrino.

Massimo Pigliucci writes, "Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true."

Could Pigliucci please share with us when he observed a proton, electron and electron antineutrino combining to form a Neutron?

CFT, the are some philosophers who have written papers that show thorough knowledge of some area of physics. I concede that. But when the conventional wisdom is wrong, I worry about the whole profession.

Drop a drop of food coloring in a fish tank. The color will disperse over time.

Massimo Pigliucci quotes, "Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true."

So does Pigliucci state that at the microscopic level, the food coloring will have just as much chance converging to a point, as it does dissipating and expanding?

What does Pigliucci mean by "microscopic level"? He is welcome to try a drop of any size. Let us call this the "Pigliucci drop." According to Pigliucci, the tiny "Pigliucci drop" of food coloring will have as much chance of contracting and converging as it does expanding and dissipating. Thus, it would maintain its size?

We invite Pigliucci to perform his experiment and let us know how small a drop of food coloring must be for it to have as much chance of dissipating as it does converging.

Measure the position of an electron in free space, and you can determine where it is. After the measurement, as time passes, the uncertainty of the electron's position will increase.

Massimo Pigliucci writes, "Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true."

I invite Pigliucci to conduct an experiment where after the electron's position is measured in free space, the uncertainty in its position decreases instead of increasing over time. This simple experiment could be a Nobel Prize!

As Massimo Pigliucci speaks on his brand new iPhone 7 cell phone, the photons of electromagnetic waves propagate outwards as expanding spherically-symmetric waves of probability.

Massimo Pigliucci writes, "Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true."

I invite Pigliucci to speak on his iPhone in such a manner that instead of expanding as spherically-symmetric waves of probability, the photons converge on his phone as SHRINKING spherically-symmetric waves of probability.

Perhaps Pigliucci could say his sentences backwards, and this would happen?

Massimo Pigliucci writes, "Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true."

Pigliucci is stating that when the nuclear fusion is just beginning in a nuclear bomb, and is all occurring on a microscopic level, there is an equal chance that the bomb will never explode as there is that it will explode.

As a cannonball falls to the earth, it gets closer and closer. At some point, the distance between it and the earth is microscopic.

Massimo Pigliucci writes, "Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true."

According to Pigliucci, when a Cannonball gets close enough to the earth, it has an equal chance of rising as it does falling.

Pigliucci could perhaps be the next Galileo, demonstrating the rising Pigliucci Cannonball from the leaning Tower of Pisa, where he drops it, and it returns to his hand.

And too, in order to fall after he drops it, the Cannonball will be a microscopic distance from his hand--a region wherein time can run backwards. Thus, there is a good chance that the Pigliucci Cannonball will never fall, and just hover.

Yes, but in physics there is no such thing as a perfectly elastic cannonball. :) Due to the Second Law of Thermodynamics, energy will be dissipated. OF course Pigliucci claims that the Second Law of Thermodynamics is not official, so we invite him to demonstrate the perfectly-elastic bounce of the Pigliucci Cannonball. :)

Consider two entangled photons A and B. When the spin of A is measured along the Z axis, the spin of B is found to have the opposite sign. The photons are no longer entangled after the measurement.

As Pigliucci states that there is no arrow of time for microspic objects such as photons, he could surely conduct an experiment wherein he re-entangles the photons, un-measures their spins, and returns them through the beam-splitter to their source.

1. There is a very widespread view among laymen, and unfortunately among philosophers too, that causality requires a passage of time. As just one example: In the domino effect, the fall of one domino leads to the fall of another domino only after an elapse of time.

In fact, all their examples wherever causality is operative, are of the following kind:

"If something happens then something else happens (necessarily)."

Now, they interpret the word `then' to involve a passage of time. (Then, they also go on to worry about physics equations, time symmetry, etc., but in my view all these are too advanced considerations; they are not fundamental or even very germane at the deepest philosophical level.)

2. However, it is possible to show other examples involving causality, too. These are of the following kind:

"When something happens, something else (necessarily) happens."

Here is an example of this latter kind, one from classical mechanics. When a bat strikes a ball, two things happen at the same time: the ball deforms (undergoes a change of shape and size) and it "experiences" (i.e. undergoes) an impulse. The deformation of the ball and the impulse it experiences are causally related.

Sure, the causality here is blatantly operative in a symmetric way: you can think of the deformation as causing the impulse, or of the impulse as causing the deformation. Yet, just because the causality is symmetric here does not mean that there is no causality in such cases. And, here, the causality operates entirely without the dimension of time in any way entering into the basic analysis.

Here is another example, now from QM: When a quantum particle is measured at a point of space, its wavefunction collapses. Here, you can say that the measurement operation causes the wavefunction collapse, and you can also say that the wavefunction collapse causes (a definite) measurement. Treatments on QM are full of causal statements of both kinds.

3. There is another view, concerning causality, which is very common among laymen and philosophers, viz. that causality necessarily requires at least two separate objects. It is an erroneous view, and I have dealt with it recently in a miniseries of posts on my blog; see https://ajitjadhav.wordpress.com/2017/05/12/relating-the-one-with-the-many/.

4. Notice, the statement "when(ever) something happens, something else (always and/or necessarily) happens" is a very broad statement. It requires no special knowledge of physics. Statements of this kind fall in the province of philosophy.

If a layman is unable to think of a statement like this by way of an example of causality, it's OK. But when professional philosophers share this ignorance too, it's a shame.

5. Just in passing, noteworthy is Ayn Rand's view of causality: http://aynrandlexicon.com/lexicon/causality.html. This view was basic to my development of the points in the miniseries of posts mentioned above. ... May be I should convert the miniseries into a paper and send it to a foundations/philosophy journal. ... What do you think? (My question is serious.)

Thanks for highlighting the issue though; it's very deeply interesting.